Process for purifying n-alkylpyrrolidones

ABSTRACT

Process for purifying N-alkylpyrrolidones which due to a previous use comprise at least one of the impurities of the formula I or II 
     
       
         
         
             
             
         
       
     
     where R is hydrogen or a C1-C20-alkyl group,
 
wherein
         a basic compound is added to the N-alkylpyrrolidone to be purified and the temperature of the mixture is at least 80° C. not more than 20 minutes after addition of the basic compound and   N-alkylpyrrolidone is distilled off from the mixture obtained.

The present invention relates to a process for purifyingN-alkylpyrrolidones which due to a previous use comprise at least one ofthe impurities of the formula I or II

where R is hydrogen or a C1-C20-alkyl group.

In this process,

-   -   a basic compound is added to the N-alkylpyrrolidone to be        purified and the temperature of the mixture is at least 80° C.        not more than 20 minutes after addition of the basic compound        and    -   the N-alkylpyrrolidone is distilled off from the mixture        obtained.

The present invention further relates to a process for the use,purification and reuse of N-alkylpyrrolidones, wherein

-   a) a polymer is dissolved in an N-alkylpyrrolidone,-   b) electrodes of lithium ion accumulators are produced using this    polymer and a contaminated N-alkylpyrrolidone comprising at least    one of the impurities of the formula I or II

-    where R in formulae I and II is hydrogen or a C1-C20-alkyl group,    is recovered,-   c) a basic compound is added to the recovered N-alkylpyrrolidone and    the temperature of the mixture is increased to at least 80° C. not    more than 20 minutes after addition of the basic compound,-   d) the N-alkylpyrrolidone is distilled off from the mixture obtained    and-   e) is reused as solvent for the production of electrodes for lithium    ion accumulators.

Polymers are frequently used for producing lithium ion accumulators,e.g. as polymer electrolyte or for producing electrodes. It is usual touse, for example, polyvinylidene fluoride for producing the electrodes.

For this purpose, polyvinylidene fluoride is usually dissolved in anorganic solvent and a suspension is produced by addition of lithiumstorage materials and optionally conductive additives. The electrode isobtained by coating a support, for example a metallic support, with thissuspension and subsequently removing the solvent. N-Alkylpyrrolidones,in particular N-methylpyrrolidone, have been found to be suitablesolvents.

The solvent can be contaminated as a result of the above use. Thecoating process introduces impurities, and by-products of the solventused are also formed as a result of the high temperatures requiredand/or the presence of oxidizing compounds. In the case ofN-methylpyrrolidone (NMP), compounds of the above formulae I and II,where R is an H atom, are formed by undesirable secondary reactions.These compounds have a boiling point similar to that of NMP and aretherefore very difficult to separate from NMP by distillation.

Only solvents having a high purity and freedom from water can be usedfor producing long-lived lithium ion accumulators. A solvent which hasbeen used once can only be reused when it has been purified and onceagain meets the demanding requirements in respect of purity and freedomfrom water.

JP 11071346 and JP 10310795 disclose purifying NMP which has been usedas solvent for polyvinylidene fluoride by addition of acidic compounds,in particular by contact with acidic solids.

WO 2010/057917 describes the purification of NMP which has been used assolvent for polyvinylidene fluoride and the production of lithium ionaccumulators. The purification is effected by addition of activatedcarbon, and the process is therefore a purification by adsorption.

The addition of alkali metal hydroxides or alkaline earth metalhydroxides to NMP has also already been described. Thus, distilling NMPin the presence of alkali metal hydroxides or alkaline earth metalhydroxides in order to prevent the formation of peroxides and stabilizethe NMP is known from JP 2004284958 and JP 2007099690.

U.S. Pat. No. 4,965,370, JP 2001354769 and JP 11349566 describe thepurification of NMP which has been used in the preparation ofpolyarylene sulfide; impurities from the polyarylene sulfide process canin each case be removed by addition of alkali metal hydroxides oralkaline earth metal hydroxides and the NMP can be reused for thisprocess.

JP 03014559 discloses the removal of N-methylsuccinimide and phenol fromNMP by addition of metal hydroxides or metal carbonates. Here too, theNMP concerned is from the polyarylene sulfide process.

WO 03/053924 describes a process for preparing NMP fromgamma-butyrolactone and methylamine. Here, metal hydroxides are used toremove excess gamma-butyrolactone by salt formation.

It was an object of the present invention to provide a process forpurifying previously used N-alkylpyrrolidone, in particular NMP, whichallows the reuse of the N-alkylpyrrolidones for any applications, inparticular for the production of lithium ion accumulators. The processshould be very simple and effective. Impurities of the above formulae Iand II which have been introduced or formed should ideally be removedcompletely by the process.

We have accordingly found the process defined above.

The process of the invention is a process for purifyingN-alkylpyrrolidones which have already been used at least once. Forexample, the N-alkylpyrrolidones could have been used as solvents.

The N-alkylpyrrolidones concerned are preferably N-alkylpyrrolidones ofthe formula A

where R is hydrogen or a C1-C20-alkyl group.

In particular, R in formula A is hydrogen or a C1-C3-alkyl group, and Ris particularly preferably hydrogen or a methyl group. If R is hydrogen,the compound concerned is N-methylpyrrolidone (NMP), and if R is amethyl group, the compound concerned is N-ethylpyrrolidone (NEP).Particular preference is given to NMP or NEP. Very particular preferenceis given to NMP.

As a result of the previous use, the N-alkylpyrrolidone comprises atleast one of the impurities of the formula I or II

where R is hydrogen or a C1-C20-alkyl group.

The compounds of the formula I and II are isomers which frequently occurin admixture. The N-alkylpyrrolidone to be purified can thereforecomprise both compounds. R is preferably hydrogen or a C1-C4-alkylgroup.

When the N-alkylpyrrolidone to be purified is NMP, R in formulae I andII is hydrogen.

When the N-alkylpyrrolidone to be purified is NEP, R in formulae I andII is a methyl group.

The content of compounds of the formulae I and II in theN-alkylpyrrolidone can be, in particular, from 0.0005 to 1 part byweight per 100 parts by weight of N-alkylpyrrolidone. In the presentprocess, particular preference is given to using an N-alkylpyrrolidonehaving a total content of compounds of the formulae I and II of from0.0005 to 0.5 part by weight, particularly preferably from 0.0005 to 0.1part by weight, per 100 parts by weight of N-alkylpyrrolidone.

Apart from the compounds of the formulae I and II, theN-alkylpyrrolidone to be purified can comprise further organic compoundsas impurities.

Further organic compounds which can be comprised as impurity are, forexample, those of the following formulae

where R is in each case hydrogen or a C1-C20-alkyl group.

Apart from the compounds of the formulae I, II and possibly III to XII,the N-alkylpyrrolidone to be purified can comprise further organiccompounds which originate from the production process for theN-alkylpyrrolidone and are not counted as impurities. Apart fromgamma-butyrolactone and the amine used for preparing theN-alkylpyrrolidone (methylamine in the case of NMP), these furtherorganic compounds are derivatives of N-alkylpyrrolidone which aresubstituted by alkyl radicals on the ring carbons. Preferred alkylradicals are methyl and ethyl groups and can be present eitherindividually or as a mixture on all ring carbons. The content of thesubstituted N-alkylpyrrolidones is generally less than 1 part by weight,in particular less than 0.5 part by weight, per 100 parts by weight ofN-alkylpyrrolidone. The content of gamma-butyrolactone is generally lessthan 0.1 part by weight, in particular less than 0.05 part by weight,per 100 parts by weight of N-alkylpyrrolidone. The content of the amineused for preparing the N-alkylpyrrolidone is generally less than 0.005part by weight, in particular less than 0.003 part by weight, per 100parts by weight of N-alkylpyrrolidone.

The N-alkylpyrrolidone to be purified generally comprises a total offrom 0.0005 to 5 parts by weight, in particular from 0.0005 to 2 partsby weight, of organic compounds of the formulae I to XII as inimpurities per 100 parts by weight of N-alkylpyrrolidone.

The N-alkylpyrrolidone to be purified can further comprise water or canbe present in admixture with water. Large amounts of water can have beenintroduced by previous uses.

The N-alkylpyrrolidone to be purified can therefore have, for example,the following composition

100 parts by weight of N-alkylpyrrolidone,a total of from 0.0005 to 5 parts by weight of organic compounds of theformulae I to XII as impurities andfrom 0.01 to 200 parts by weight of water.

In particular, the N-alkylpyrrolidone to be purified can have thefollowing composition

100 parts by weight of N-alkylpyrrolidonea total of from 0.0005 to 2.5 parts by weight of organic compounds ofthe formulae I to XII as impurities and from 0.1 to 100 parts by weightof water.

A basic compound is added to the N-alkylpyrrolidone to be purified. Inthe following, the term basic compound also encompasses mixtures ofvarious basic compounds.

The basic compound is preferably a compound which in an amount of 5parts by weight in 100 parts by weight of water (at 20° C., 1 bar) givesa pH of at least 8, particularly preferably at least 9. In the abovedefinition, it is not necessary for the 5 parts by weight of the basiccompound to dissolve completely in water; the critical thing is merelythat the pH increase occurs after addition of the basic compound.

The basic compound can be organic or inorganic compounds. Possibilitiesare, for example, alkali metal hydroxides, alkaline earth metalhydroxides, alkoxides, carbonates, carboxylates, complex hydrides,ammonia or amines. Mention may be made by way of example of LiOH, NaOH,KOH, Mg(OH)₂, Ca(OH)₂, Ca(HCO₃)₂, CaCO₃, sodium oxalate, sodiumborohydride, lithium aluminum hydride, sodium methoxide, sodiumethoxide, potassium methoxide, potassium ethoxide and the alkylaminecorresponding to the N-alkyl-substituted pyrrolidone.

Particular preference is given to inorganic compounds, in particularbases comprising alkali metal or bases comprising alkaline earth metal.

The basic compound added is very particularly preferably NaOH, KOH,sodium methoxide or potassium methoxide.

In a particularly preferred embodiment, the basic compound is sodiumhydroxide.

The basic compound can be added as such or as a solution to theN-alkylpyrrolidone to be purified. If the basic compound is added insolution, the solvent can be, for example, water or organic solvents,e.g. the respective N-alkylpyrrolidone or mixtures of the respectiveN-alkylpyrrolidone with water.

The basic compound is preferably used in amounts of from 0.01 to 10parts by weight per 100 parts by weight of the N-alkylpyrrolidone to bepurified (sum of N-alkylpyrrolidone and all organic constituents presenttherein, e.g. impurities and derivatives). It is particularly preferablyused in amounts of from 0.01 to 1 part by weight and very particularlypreferably in amounts of from 0.01 to 0.5 part by weight, in particularfrom 0.05 to 0.5 part by weight, per 100 parts by weight of theN-alkylpyrrolidone to be purified.

Not more than 20 minutes, preferably not more than 10 minutes,particularly preferably not more than 5 minutes, very particularlypreferably not more than 2 minutes, after addition of the basiccompound, the temperature of the resulting mixture ofN-alklylpyrrolidone to be purified and basic compound is at least 80° C.

The temperature is preferably at least 100° C.

The above temperature can easily be set by

-   -   combining the basic compound and the N-alkylpyrrolidone to be        purified and quickly heating the mixture obtained so that the        necessary temperature is reached within the prescribed time        (alternative 1) or    -   preheating the N-alkylpyrrolidone to be purified so that the        desired temperature prevails immediately after addition of the        basic compound (alternative 2).

A particular embodiment of alternative 2 is addition of the basiccompound directly during the distillation by means of which the purifiedN-alkylpyrrolidone is separated off.

Without the above temperature increase, solid can easily precipitate. Adisadvantage of this is that, for example, pipes and valves can becomeblocked, which makes it necessary for the plant to be shut down andrequires appropriate cleaning.

N-Alkylpyrrolidone is distilled off from the mixture. The purifiedN-alkylpyrrolidone is preferably taken off at the top of the column.

The distillation is preferably carried out at a temperature of from 50to 350° C., particularly preferably from 100 to 250° C., and a pressureof from 0.3 to 5 bar.

Suitable columns for the distillation are columns known to those skilledin the art. Preference is given to packed columns, tray columns havingsieve trays, columns having dual-flow trays, columns having bubblecapped trays or rectification columns equipped with valve trays,dividing wall columns or thin film and falling film evaporators.

The distillation is preferably carried out in the absence of oxygen. Forthe present purposes, absence means that the proportion by volume ofoxygen is less than 0.1%, in particular less than 0.01% and veryparticularly preferably less than 0.001%, based on the total volume ofthe distillation column.

The addition of the basic compound to the N-alkylpyrrolidone ispreferably carried out under an inert gas atmosphere so that the oxygencontent is reduced during the entire process. Preference is given tocarrying out the entire process in the absence of oxygen, with absenceof oxygen meaning a proportion by volume of oxygen of less than 0.1%, inparticular less than 0.01% and very particularly preferably less than0.001%, based on the total volume of the apparatus part utilized in theparticular case (vessel, distillation column).

The apparatuses used are preferably made of stainless steels, likewiseall pipes and column internals.

The process can be carried out batchwise or continuously.

When it is carried out batchwise, the basic compound can be added to aninitial charge of N-alkylpyrrolidone to be purified and the resultingmixture can be distilled batchwise.

The process is preferably carried out continuously.

For this purpose, the starting materials, i.e. the basic compound andthe N-alkylpyrrolidone to be purified, are combined continuously. Theycan, for example, be fed continuously into a reservoir and be mixedthere; the distillation can be supplied continuously from thisreservoir.

The basic compound and the N-alkylpyrrolidone to be purified can be fedas separate streams directly and continuously to the distillation.

The N-alkylpyrrolidone to be purified is preferably fed into thestripping section of the column. The basic compound can be introduced aspure substance or as solution on any tray of the distillation column.

The distillation can be carried out in one or more stages, i.e. in oneor more columns.

A dividing wall column, for example, is also suitable for asingle-stage, continuous distillation.

In a preferred embodiment, a continuous, two-stage distillation iscarried out.

For this purpose, the N-alkylpyrrolidone to be purified (preferably NMPto be purified) and the basic compound (preferably NaOH) can be fed,either in admixture or separately, to a first distillation column. Inthe first distillation column, low boilers such as methylamine, inparticular water, are preferably separated off at the top at atemperature at the bottom of from 150 to 250° C., in particular from 180to 250° C., and a pressure of from 0.1 to 5 bar, in particular from 0.3to 3 bar. In the second distillation column the bottoms from the firstdistillation column can be purified by distillation under reducedpressure, for example at a temperature at the bottom of from 80 to 180°C., in particular from 100 to 160° C., and a pressure of from 0.005 to0.5 bar, in particular from 0.01 to 0.3 bar. The purifiedN-alkylpyrrolidone can be separated off as a side offtake stream fromthe second column and the bottoms, which comprise impurities and thebasic compound or reaction products thereof, can be dischargedcontinuously.

A particular embodiment of the process comprises carrying out thedistillation of the N-alkylpyrrolidone to be purified in admixture withadditional N-alkylpyrrolidone from the production process.

N-Alkylpyrrolidone, in particular NMP, can be obtained by reaction ofgamma-butyrolactone with the corresponding amine (methylamine in thecase of NMP). This reaction gives a product stream composed ofN-alkylpyrrolidone (NMP), water, unreacted starting materials(gamma-butyrolactone and methylamine) and possibly by-products. Thisproduct stream is worked up by distillation. The N-alkylpyrrolidone tobe purified can be mixed in any amount with this product stream. TheN-alkylpyrrolidone to be purified can comprise the basic compound evenbefore being combined with the product stream. As an alternative, it isnaturally also possible to add the basic compound to the product stream,and in this case the basic compound is added to the N-alkylpyrrolidoneto be purified only by mixing with the product stream.

After the distillation, the purified N-alkylpyrrolidone comprises, per100 parts by weight of N-alkylpyrrolidone,

-   -   a total of less than 0.5 part by weight of the impurities        III-XII    -   less than 0.05 part by weight of water and    -   a total of less than 0.01 part by weight of compounds of the        formulae I and II.

In particular, the purified N-alkylpyrrolidone comprises, per 100 partsby weight of N-alkylpyrrolidone,

-   -   a total of less than 0.1 part by weight of the impurities        III-XII    -   less than 0.03 part by weight of water and    -   a total of less than 0.0005 part by weight of compounds of the        formulae I and II.

The purified N-alkylpyrrolidone comprises, in particular, less than 5%by weight, preferably less than 1% by weight, particularly preferablyless than 0.1% by weight, of the total amount of the compounds of theformulae I and II originally comprised in the contaminatedN-alkylpyrrolidone.

The purified N-alkylpyrrolidone obtained is, due to its high purity,once again suitable for the production of electrodes for lithiumaccumulators.

EXAMPLES

The following examples serve to illustrate the invention. The contentsof impurities indicated in the examples have been determined by gaschromatography (GC instrument HP6890, FID detector, nitrogen carrier gasat 1.0 mL/min (const. flow); Split Ratio 1:50; column RTX-1, 30 m, 0.32mm, 1.0 μm film; temperature program: start at 80° C., then 5° C./min to140° C., then 5° C./min to 200° C. and 10 min isothermal, then 10°C./min to 340° C. and 8 min isothermal). Water values were determined byKarl-Fischer titration. NMP and the respective impurities were used asindicated in the examples. All working steps were carried out in cleanedapparatuses under a nitrogen atmosphere; the absence of foreigncompounds was monitored by GC before commencement of the experiments.

Comparative Example 1

A contaminated NMP comprising 0.5% of water and 0.031% of impurities ofthe formulae I-XII (impurity of the formulae I and II, 0.004%; impurityof the formula III, 0.017%; impurity of the formulae IV-VI, 0.003%;impurity of the formulae VII-IX, 0.002%; impurity of the formulae X-XII,0.005%) was admixed with 0.1% of NaOH (calculated as pure NaOH, used as30% strength aqueous solution) and pumped into a tank. After a residencetime of 4 hours at a temperature of 15° C., about 100 kg of aprecipitate precipitated from an initial amount of 23.5 metric tons ofNMP. For the subsequent two-stage distillation, the contents of the tankwere continuously conveyed via a riser tube in an amount of 500 L/h intothe first valve tray column (40 trays, inlet on tray 21) and the feedstream was stopped after introduction of 23.3 metric tons, so that thesolid remained in the tank. The distillation was carried out attemperatures at the bottom in the range from 180 to 185° C., a pressureat the top of the column of 530 mbar and a reflux to offtake ratio ofabout 1:1. The bottoms from the column were conveyed into the secondvalve tray column (40 trays, inlet on tray 21) and distilled attemperatures at the bottom in the range from 165 to 170° C. and apressure at the top of the column of 220 mbar, with the product beingtaken off at the top. The sodium content of the feed to the second valvetray column was in the range from 0.0005 to 0.001%; the distillatecontinued to have the impurities of the formulae I and II in an amountof 0.002% and impurity of the formula IV-VI in an amount of 0.001% andwas free of the impurity of the formula III; the distillation yield was90% and the water content of the product was 0.005%.

Comparative Example 2

A contaminated NMP comprising 1.0% of water and 0.074% of impurities ofthe formulae I-XII (impurity of the formulae I and II, 0.003%; impurityof the formula III, 0.035%; impurity of the formulae IV-VI, 0.002%;impurity of the formulae VII-IX, 0.032%; impurity of the formulae X-XII,0.002%) was admixed with 0.1% of NaOH (calculated as pure NaOH, used as30% strength aqueous solution) and pumped into a container. About 0.1 kgof a precipitate precipitated from an initial amount of 30 kg of NMPwithin a residence time of 6 hours at a temperature of 25° C. For thesubsequent distillation, the contents of the container were transferredinto the pot of a distillation apparatus and fractionated batchwise. Thesolid remained in the container. The distillation was carried out in acolumn (Sulzer CY Packung, 13 lengths of 0.63 m, 1 of 0.30 m) having arunback divider at temperatures at the bottom in the range from 130 to140° C. and pressures in the range from 1000 mbar (low boiler removal)and 90 mbar at the top at a reflux to offtake ratio of 50:1. A total of30 distillate fractions were taken off. The first five fractionsobtained at temperatures at the top of 55-118° C. comprisedpredominantly water (>98%). The subsequent fractions 6-24 (altogetherabout 21 kg) were obtained at 100 mbar and a temperature at the top of122-126° C. These comprised water in an amount of less than 0.1%, NMP inpurities above 99.8% but still the impurities of the formulae I and II,0.001% and of the formulae IV-VI, 0.001%, present in the initial charge.From fraction 25 (altogether about 4 kg), the undesirable impurities ofthe formulae I-XII could no longer be detected by GC analysis: thedistillation yield was 13% and the water content of the product was0.005%.

Examples According to the Invention Example 1

The feed mixture from comparative example 1 was admixed continuouslywith 0.1% of NaOH (calculated as pure NaOH, used as 30% strength aqueoussolution) by means of a static mixer at a residence time of 30 secondsat a feed rate of the contaminated NMP of 500 liters (l)/hour (h) and atemperature of 15° C. This feed stream was, in contrast to comparativeexample 1, introduced directly into the first valve tray column (inletof tray 21) and heated there to above 150° C. within 2 minutes. Theformation of a solid was not observed. The distillation was carried outin two stages in a manner analogous to comparative example 1. The sodiumcontent in the feed to the second valve tray column was in the rangefrom 0.05 to 0.06%, the distillation yield was 90%, the impurities ofthe formulae I-XII could no longer be detected in the product by GCanalysis and the water content was 0.005%.

Example 2

The feed mixture from comparative example 2 was mixed with 0.1% of NaOH(calculated as pure NaOH, used as 30% strength aqueous solution) bymeans of a static mixer at a residence time of 30 seconds and heated to95° C. within 10 minutes by means of a preheater, with formation of asolid not being observed. The mixture was transferred at a feed rate of5 l/min into the pot of a distillation apparatus and directlyfractionated batchwise in a manner analogous to comparative example 2(heating to 130° C. within a further 20 minutes). A total of 20distillation fractions were taken. The first five fractions obtained attemperatures at the top of 55-118° C. comprised predominantly water(>98%). The subsequent fractions 6-20 (altogether about 25 kg) wereobtained at 100 mbar and a temperature at the top of 122-126° C. Thesecomprised water in an amount of less than 0.1%, NMP in purities above99.8% and the undesirable impurities of the formulae I-XII could nolonger be detected by GC analysis. The distillation yield was 83% andthe water content of the product was 0.005%.

Example 3

The feed mixture from comparative example 2 was mixed with 0.15% ofpotassium methoxide (KOMe for short, calculated as pure KOMe, used as25% strength methanolic solution) by means of a static mixer at aresidence time of 30 seconds and heated to 95° C. within 10 minutes bymeans of a pressure-resistant preheater, with formation of a solid notbeing observed. The mixture was transferred at a feed rate of 10 l/mininto the pot of a distillation apparatus and directly fractionatedbatchwise in a manner analogous to comparative example 2 (heating to130° C. within a further 20 minutes). A total of 20 distillationfractions were taken. The first five fractions obtained at temperaturesat the top of 55-118° C. comprised predominantly water (>98%) and asmall proportion of methanol. The subsequent fractions 6-20 (altogetherabout 25 kg) were obtained at 100 mbar and a temperature at the top of122-126° C. These comprised water in an amount of less than 0.1%, NMP inpurities above 99.8% and the undesirable impurities of the formulaeI-XII and methanol could no longer be detected by GC analysis.

Example 4

The feed mixture (500 milliliters) from comparative example 1 was heatedwith 1% of monomethylamine (calculated as pure amine, used as 40%strength aqueous solution) to 100° C. in a steel autoclave over a periodof 15 minutes and then heated further at 200° C. for 6 hours, with apressure of about 20 bar being built up. The solids-free reaction outputwas subsequently distilled in a laboratory apparatus (standarddistillation setup with Vigreux column) under conditions analogous tocomparative example 2. About 90% of NMP in the desired purity(monomethylamine content<0.002%) were obtained, with the predominantpart of the remainder being contaminated only by water contents whichwere too high (>0.1%).

1. A process for purifying N-alkylpyrrolidones which due to a previoususe comprise at least one of the impurities of the formula I or II

where R is hydrogen or a C1-C20-alkyl group, wherein a basic compound isadded to the N-alkylpyrrolidone to be purified and the temperature ofthe mixture is at least 80° C. not more than 20 minutes after additionof the basic compound and N-alkylpyrrolidone is distilled off from themixture obtained.
 2. The process according to claim 1, wherein theN-alkylpyrrolidone is N-methylpyrrolidone.
 3. The process according toclaim 1, wherein the basic compound is a compound which in an amount of5 parts by weight in 100 parts by weight of water (at 20° C., 1 bar)gives a pH of at least
 8. 4. The process according to claim 1, whereinthe basic compound is selected from alkali metal hydroxides or alkalineearth metal hydroxides.
 5. The process according to claim 1, wherein thebasic compound is added in amounts of from 0.01 to 10 parts by weightper 100 parts by weight of N-alkylpyrrolidone.
 6. The process accordingto claim 1, wherein the temperature of the mixture is at least 100° C.not more than 5 minutes after addition of the basic compound.
 7. Theprocess according to claim 1, wherein the basic compound is added duringthe distillation.
 8. The process according to claim 1, wherein thedistillation is carried out at a temperature of from 100 to 300° C. anda pressure of from 0.005 to 10 bar.
 9. The process according to claim 1,wherein the distillation is carried out in the absence of oxygen. 10.The process according to claim 1, wherein the distillation is carriedout continuously.
 11. The process according to claim 1, wherein thedistillation of the N-alkylpyrrolidone to be purified is carried out inadmixture with additional N-alkylpyrrolidone from the productionprocess.
 12. The process according to claim 1, wherein the purifiedN-alkylpyrrolidone obtained after the distillation comprises, per 100parts by weight of N-alkylpyrrolidone, a total of less than 0.5 part byweight of other constituents, less than 0.05 part by weight of water anda total of less than 0.005 part by weight of compounds of the formulae Iand II.
 13. A process for the use, purification and reuse ofN-alkylpyrrolidones, wherein a) a polymer is dissolved in anN-alkylpyrrolidone, b) electrodes of lithium ion accumulators areproduced using said polymer and a contaminated N-alkylpyrrolidonecomprising at least one of the impurities of the formula I or II

 where R is hydrogen or a C1-C20-alkyl group, is recovered, c) a basiccompound is added to the recovered N-alkylpyrrolidone to be purified andthe temperature of the mixture is increased to at least 80° C. not morethan 20 minutes after addition of the basic compound, d) theN-alkylpyrrolidone is distilled off from the mixture obtained and e) theN-alkylpyrrolidone is reused as solvent for the production of electrodesfor lithium ion accumulators.